Referring to FIGS. 2 and 3, a description will now be given of one illustrative method of machining (specially finishing) a mounting surface of a motor frame body, and one illustrative motor for which the method of machining is applied.
The motor includes a stator 14 having a stator core 12 with a winding 10 wound thereon, a rotor 20 having a rotor core 18 secured to a shaft 16, and a frame body (i.e., a front frame 22 and a rear frame 24) to fixedly hold the stator core 12 therebetween in an axial direction.
The stator core 12 and the rotor core 18 include a laminated product of magnetic thin film such as silicon steel plate. The front frame 22 and the rear frame 24 include ring-like members made of metal such as aluminum. A ring-like front bearing housing 30 and a ring-like rear bearing housing 32 are respectively fixedly disposed in central openings 26 and 28 provided in the frames 22 and 24. Both the bearing housings 30 and 32 rotatably support the shaft 16 through a front bearing 34 and a rear bearing 36. It is thereby possible to rotatably support the rotor 20 by the front frame 22 and the rear frame 24 while forming fine clearances between an outer peripheral surface of the rotor core 18 and an inner peripheral surface of the stator core 12.
The central opening 26 in the front frame 22 has a smaller inner diameter than an outer diameter of the rotor 20, and the central opening 28 in the rear frame 24 has a larger inner diameter than the outer diameter of the rotor 20. Therefore, in the motor assembling process, the stator 14 is first fixed to the front frame 22 on which the front bearing housing 30 is previously mounted, and the rear frame 24 with the rear bearing housing removed therefrom. Subsequently, the rotor 20 with the front bearing 34 and the rear bearing 36 secured to the vicinity of both ends of the shaft 16 is inserted into the stator 14 through the central opening 28 in the rear frame 24. After the front bearing 34 is inserted into the front bearing housing 30, the rear bearing housing 32 is inserted into the central opening 28 in the rear frame 24 to be fixed to the rear frame 24 while inserting the rear bearing 36 into the rear bearing housing 32. The front bearing housing 30 serves as a spacer to smooth a difference in diameter between the central opening 26 in the front frame 22 and the front bearing 34. Thus, the front bearing housing 30 may appropriately be replaced or omitted depending upon a variation in dimension of the front bearing, which is required according to an operating specification of the motor.
The front frame 22 is disposed on the side of a load of the shaft 16, and is provided with a flange 38 radially outwardly extending from its front end spaced apart from the stator core 12. The flange 38 forms a mounting area for mounting the motor on an object, and has through-holes 40 into which mounting bolts (not shown) are inserted. Further, the front frame 22 is provided with a cylindrical socket portion 42 axially forwardly extending beyond the flange 38. The socket portion 42 engages a peripheral edge of a motor receiving hole provided in the object (not shown) on which the motor is mounted, resulting in relative alignment (such as centering) between the shaft 16 and a driven body.
In the front frame 22, an axial front end surface 38a of the flange 38 and a radial outer peripheral surface 42a of the socket portion 42 are machined for a surface finish to serve as mounting surfaces to make direct contact with the surface of the object on which the motor is mounted, and enable mounting of the motor with a relationship appropriately maintained between the shaft 16 and the driven body. In the prior-art motor assembling/manufacturing process, the finishing operation has been carried out according to the following procedure.
As shown in FIG. 3, the front frame 22 and the rear frame 24 are fixed to the stator core 12 with the winding 10 mounted thereon through a plurality of tie-bolts 44. Subsequently, the stator core 12 is mounted to a lathe (not shown) while supporting an inner peripheral portion of the stator core 12 by a jig (not shown). Then, the front frame 22 and the rear frame 24 fixed on the stator core 12 are rotated about an axis of the stator core 12 so that the mounting surfaces of the front frame 22 and the rear frame 24 are able to be cut by the lathe. Specifically, a concurrent surface finish is made to the axial front end surface 38a of the flange 38 and the radial outer peripheral surface 42a of the socket portion 42 in the front frame 22, and to an inner peripheral surface 26a of the central opening 26 in the front frame 22 and a mounting end surface 26b, which serve to support the front bearing housing 30, in the vicinity of the opening. It is also possible to finish a peripheral surface 28a of the central opening 28 in the rear frame 24 and a mounting end surface 28b in the vicinity of the opening so as to support the rear bearing housing 32 at an appropriate position.
The front bearing housing 30 is finished, in different steps, for an outer peripheral surface 30a to make contact with the inner peripheral surface 26a of the central opening 26 in the front frame 22, an end surface 30b to make contact with the mounting end surface 26b of the front frame 22, and an inner peripheral surface 30c and an end surface 30d to make contact with an outer peripheral surface 46a and a bearing end surface 46b of an outer ring 46 for the front bearing 34. Further, in different steps, the rear bearing housing 32 is finished for an outer peripheral surface 32a and an end surface 32b to make contact with the inner peripheral surface 28a and the mounting end surface 28b of the central opening 28 in the rear frame 24, and an inner peripheral surface 32c to make contact with an outer peripheral surface 48a of an outer ring 48 for the rear bearing 36. For both the bearings 34 and 36, their inner rings 50 and 52 are fixed on the shaft 16 in advance. Further, turning is made in advance to a peripheral surface of the shaft 16 with respect to center holes 54 provided in its both ends.
After the completion of finishing of each member, the motor assembling process is started. That is, the front bearing housing 30 is fitted into the central hole 26 in the front frame 22 with a clearance to be fixed to the front frame 22 by bolts 56. Next, the rotor 20 is inserted into the stator 14 from the side of the rear frame 24 to mount the front bearing 34 fixed to the shaft 16 on the front bearing housing 30 through clearance fitting. Subsequently, while mounting the rear bearing 36 fixed to the shaft 16 on the rear bearing housing 32 through clearance fitting, the rear bearing housing 32 is fitted into the central opening 28 in the rear frame 24 with a clearance to be fixed to the rear frame 24 through bolts 58. Thus, the motor assembling process is completed.
In the above method of finishing, the mounting surfaces (i.e., the front end surface 38a and the outer peripheral surface 42a of the front frame 22) are machined with reference to an axis of the stator 14. Therefore, it is essential to accurately align the axis of the stator 14 with an axis of the rotor 20 when the rotor 20 is mounted on the front frame 22 and the rear frame 24. In the motor assembled after the completion of machining of the mounting surfaces, the stator axis and the rotor axis may slightly be deviated from one another. In this case, even if the mounting surfaces are finished with high accuracy, and the motor is accurately mounted on the object, it is difficult to obtain the appropriate relationship between the shaft 16 of the motor and the driven body. In particular, in case of a so-called direct-coupled machine structure in which a main shaft of a machine tool is directly coupled to the shaft 16 of the motor (i.e., when the main shaft of the machine tool serves as the driven body), serious damage is caused to reliability of the operation of the machining unless the shaft 16 and the main shaft are accurately positioned (i.e., aligned).
In the assembly of the motor, in order to accurately align the stator axis with the rotor axis, it is sufficient to respectively tightly fit the bearings 34 and 36 into the bearing housings 30 and 32, and the bearing housings 30 and 32 into the frames (the front frame 22 and the rear frame 24). However, as shown in FIG. 3, the bearings 34 and 36 are fixed in advance on the shaft 16 of the rotor 20. Hence, in the fixed state, it is extremely difficult to tightly fit the bearings 34 and 36 into the bearing housings 30 and 32, and tightly fit the bearing housings 30 and 32 into the front frame 22 and the rear frame 24, resulting in a significant deterioration of assembling workability.
Therefore, in the prior art, the clearance fitting is provided between the bearings 34, 36 and the bearing housings 30, 32, and between the bearing housings 30, 32 and the frame bodies 22, 24. As a result, such slight misalignment as mentioned above tends to be easily generated between the stator axis and the rotor axis.
Further, the above finish is made to the mounting surfaces in a state in which the rotor 20 is not mounted on the frame body (the front frame 22 and the rear frame 24). Consequently, for assembly of the motor, it is necessary to mount the rotor 20 on the frame body after the completion of the finish. For this purpose, in the motor, the central opening 28 having a larger diameter than an outer diameter of the rotor 20 is formed at least one (the rear frame 24 in the embodiment of FIGS. 2 and 3) of the frame bodies (the front frame 22 and the rear frame 24) to be disposed at axial both ends of the stator 14 so as to insert the rotor 20 into the stator 14, and the discrete bearing housing 32 is mounted in the central opening 28. However, in the motor finished as described above, there are problems of increases in the number of parts and the number of steps in the assembling process.